Composition and process for treating tinplate

ABSTRACT

To impart an excellent corrosion resistance and adherence to the surface of tinplate while avoiding the production of sludge in the treatment bath during continuous treatment, a bath is used that contains phosphate ions, from 0.1 to 5.0 g/L of chelating agent, and tin ions; has a pH from 2.0 to 4.5; is essentially free of oxidizing agent and ferric ion; and has an oxidation-reduction potential of ≦450 mV more oxidizing than a silver-saturated silver chloride reference electrode.

TECHNICAL FIELD

The invention relates to a phosphate containing composition (oftendenoted hereinafter as a "bath" for brevity) for treating the surface oftinplate (i.e,, tin-plated steel) and to a treatment process fortinplate. More specifically, the present invention relates to animprovement of a treatment that is already used, prior to the paintingor printing of surfaces of tinplate sheet, strip, and formed objects,e.g., cans and the like, to provide such surfaces with an excellentcorrosion resistance and paint adherence. In particular, the treatmentbath and treatment process of the present invention are well adapted fortreating surfaces of tinplate that has been formed by DI (i.e.,drawing-and-ironing) processing. Thus, the present invention concerns anovel technology for treating tinplate surfaces, a technology that maybe used to provide tinplate surfaces with an excellent corrosionresistance and paint adherence, but which is free or very nearly free ofthe insoluble salts (hereinafter referred to as "sludge") that areproduced by the tin ions and iron ions that elute into the bath duringtreatment. This sludge reduces the productivity of tinplate surfacetreatment lines.

BACKGROUND ART

The cleaning and surface treatment of tinplate is frequently conductedby a spray process. For example, the surface treatment equipment fortinplate DI can is generally called a washer. Molded DI can is invertedand continuously treated in the washer with a cleaning bath and asurface treatment bath. Existing washers normally use 6 steps(pre-cleaning, cleaning, water wash, surface treatment, water wash, andwash with de-ionized water), and treatment is conducted entirely byspraying.

Compositions of phosphate ion, tin ion, and oxidizing agent are alreadyknown as surface treatment baths for tinplate DI can. As discussed bythe present inventors in Nihon Parkerizing Giho, 89, No. 2, page 6, themechanism of conversion film formation by these components consists oftin and iron elution reactions (anodic reactions) and the precipitationof insoluble phosphate salts (cathodic reaction).

Furthermore, in Japanese Patent Application Laid Open [Kokai orUnexamined] Number Hei 1-100281 [100,281/1989]), there has already beenproposed a composition for the purpose of inverting the tin-ironpotential in the conversion bath, i.e., the tin region becomes the anodeand the iron region becomes the cathode. This particular inventionconsists of a conversion coating bath for the treatment of metalsurfaces. This bath has a pH of 2 to 6 and contains 1 to 50 grams perliter (hereinafter often abbreviated "g/L") of phosphate ions, 0.2 to20.0 g/L of oxyacid ions, 0.01 to 2.0 g/L of tin ion, and 0.01 to 5.0g/L of condensed phosphate ions. Treatment with this conversiontreatment bath forms a highly corrosion-resistant, highly paint-adherentphosphate film on the surface of tinplate DI can. The oxyacid ion is anoxidizing agent that functions to oxidatively remove the hydrogen thatis produced by the anodic reactions.

When the aforesaid invention is practiced on a continuous basis, it isin fact capable of initially providing an excellent surface treatment.However, it has been found that the referenced invention graduallygenerates a phosphate salt sludge, which is produced by the reaction ofthe phosphate ions present in the bath with the tin ions and iron ionsthat elute from the tinplate. It has also been determined that iron ionselute from the tinplate in the divalent state; that gradual oxidation bythe oxidizing agent (oxyacid ion, etc.) produces the trivalent state inthe surface treatment bath at a level of approximately 0.05 g/L; andthat this is the cause of sludge production.

This sludge can cause problems by adhering to the tinplate surface anddegrading the paint adherence. In addition, the sludge can clog thepiping and nozzles of the spray equipment and can thereby prevent a highquality surface treatment. This has necessitated the implementation ofperiodic maintenance in order to clean the piping and nozzles of thespray equipment and has resulted in unstable quality characteristics.Since productivity enhancements and improvements in quality stabilityhave recently become critical issues, a surface treatment bath isdesired that carries a reduced cleaning burden and that offers stablequality characteristics, i.e., that is free of sludge production in thebath even during continuous service.

DISCLOSURE OF THE INVENTION Problem(s) to Be Solved by the Invention

Accordingly, the present invention takes as its object the introductionof a bath and process for treating tinplate surfaces that solves theproblems described above and that enhances quality stability and leadsto improvements in productivity (easy maintenance and the like).

SUMMARY OF THE INVENTION

As a result of extensive research into the problems described above, itwas determined that sludge production is particularly significantlyinfluenced by the oxidation state (divalent or trivalent) in thetreatment composition of the iron ions present therein, which normallyelute from the tinplate during treatment with the composition. Withrespect to a bath for treating tinplate surfaces that comprises,preferably consists essentially of, or more preferably consists of,water, acidity, phosphate ions, chelating agent, and tin ions, it wasalso determined that an excellent corrosion resistance and paintadherence could be obtained without sludge production--even duringcontinuous treatment--by such a bath for treating tinplate surfaces thathas a pH in the range of 2.0 to 4.5 and a concentration of chelatingagent in the range of 0.1 to 5.0 g/L and that essentially does notcontain ferric iron or an oxidizing agent sufficiently strong to oxidizeferrous to ferric ions. The present invention was achieved based onthese findings.

In addition, the iron ions eluting from tinplate often undergospontaneous oxidation to the trivalent state when the surface treatmentprocess employs the surface treatment bath on a continuous basis. Withthe objective of maintaining the iron ions in the divalent state, theuse of the oxidation-reduction potential to monitor the oxidation stateof the iron ions was therefore examined. As a result, with respect tothe treatment of tinplate surfaces by contacting tinplate with an acidicsurface treatment bath that contains at least phosphate ion, chelatingagent, and tin ion, a method for treating tinplate surfaces wasdiscovered whose characteristic features are a pH in the surfacetreatment bath in the range of 2.0 to 4.5 and control of theoxidation-reduction potential of the surface treatment bath to ≦450 mVby the addition of reducing agent on an as-required basis. The presentinvention was also achieved based on this discovery. The structure ofthe present invention is explained in detail below.

DESCRIPTION OF PREFERRED EMBODIMENTS

Phosphoric acid (H₃ PO₄), sodium phosphate (Na₃ PO₄), and the like canbe used to provide the phosphate ion, and this component should be usedin quantities sufficient to bring about tin phosphate precipitation. Thereactivity is low when phosphate ion is present at less than 1 g/L, andthis prevents satisfactory formation of the coating under ordinarytreatment conditions. While a good quality coating is formed at valuesin excess of 30 g/L, the corresponding high cost of the treatment bathbecomes economically disadvantageous. Thus, the phosphate ion is presentpreferably in the range of 1 to 30 g/L and more preferably in the rangeof 4 to 8 g/L.

The present invention requires that the bath contain chelating agent ina quantity sufficient to bring about a satisfactory etching, selectiveconversion film formation on exposed iron regions, and a satisfactorytin ion stabilization. Preferred chelating agents that meet theserequirements are exemplified by condensed phosphate ions, tartaric acid,oxalic acid, and citric acid. Particularly preferred chelating agentscomprise at least one selection from the condensed phosphate ions. Thisis because the condensed phosphate ions gradually decompose tophosphoric acid and therefore have little to no adverse effect on wastewater treatment. The acid or salt can be used to provide condensedphosphate ion. For example, pyrophosphoric acid (H₄ P₂ O₇), sodiumpyrophosphate (Na₄ P₂ O₇), and so forth can be used to providepyrophosphate ion. The etching activity is weak and film formation isunsatisfactory at a chelating agent concentration of less than 0.1 g/L.On the other hand, the etching activity is too strong and thefilm-formation reactions are inhibited at more than 5 g/L of chelatingagent. The chelating agent content therefore preferably falls in therange of 0.1 to 5 g/L and particularly preferably falls in the range of0.2 to 1.0 g/L.

Since tinplate DI can has been subjected to DI processing, its surfacepresents both tin-plated regions and iron regions that have been exposedby the processing, and the corrosion resistance is generally poor whenlarge areas of iron are exposed. For this reason, the generation ofuniform coverage of the exposed iron regions by the conversion coatingis a crucial issue from the standpoint of improving the corrosionresistance. Because the surface treatment bath of the present inventioncontains a chelating agent, it is able to selectively and uniformlycover the exposed iron regions with a conversion coating, whereas a verypoor conversion is produced at these exposed iron regions in the absenceof chelating agent. This makes possible the production of a highlycorrosion-resistant conversion film. Moreover, the chelating agent andparticularly the condensed phosphates function to stabilize the elutedtin ions in the bath and therefore also act to inhibit sludgeproduction.

The tin ions can be supplied by tin metal or a tin salt, for example,tin chloride, but the tin source is not specifically restricted. In thecase of continuous treatment, supplemental additions are notspecifically required due to elution of tin ion from the tinplate. Thetin ion content should be selected so as to yield the formation of asatisfactory tin phosphate coating, and preferably falls into the rangeof 0.01 to 2.0 g/L, more preferably into the range of 0.1 to 1.0 g/L,and particularly preferably into the range of 0.2 to 0.6 g/L. The rangeof 0.01 to 2.0 g/L yields a highly corrosion resistant film and avoidsthe precipitation of sludge.

The pH of the treatment bath should be maintained at 2.0 to 4.5. Strongetching and an inhibition of film formation are obtained at below 2.0.The anodic reaction conditions suffer from substantial deteriorationwhen the pH exceeds 4.5 because the development of the anodic reactionsis inhibited due to the essential absence of oxidizing agent from thetreatment bath in accordance with the present invention. Accordingly,the pH must be held in the range of 2.0 to 4.5, and is preferably heldin the range of 2.5 to 3.5 and more preferably in the range of 2.7 to3.3. The pH may be adjusted through the use of an acid such asphosphoric acid, sulfuric acid, and the like or through the use of analkali such as sodium hydroxide, sodium carbonate, ammonium hydroxide,and the like.

A characteristic feature of the treatment bath in accordance with thepresent invention is that essentially it contains neither ferric ironions nor any oxidizing agent that will oxidize any substantial amount offerrous iron ions to ferric iron ions. Preferably, the concentration offerric ions in any surface treatment bath according to this invention isnot greater than 7 mg/L, more preferably not greater than 3 mg/L, stillmore preferably not greater than 2.0 mg/L, or most preferably notgreater than 1.1 mg/L.

Although prior surface treatment baths have contained oxidizing agent,the surface treatment bath in accordance with the present inventionessentially does not contain an oxidizing agent such as oxyacid ion orthe like, that is, does not contain oxidizing agent which substantiallyremoves the hydrogen produced by anodic reactions. Given that trivalentiron ion facilitates the occurrence of sludge precipitation, the reasonfor omitting the oxidizing agent is that the presence of oxidizing agentleads to a condition in which both divalent and trivalent iron ions arepresent.

The absence of oxidizing agent from tinplate surface treatment baths hasheretofore resulted in Unstable conversion characteristics and inparticular in an inability to obtain a uniform conversion at exposediron regions, and for these reasons the absence of oxidizing agent hasheretofore been considered undesirable. However, the continuousexecution of conversion while still maintaining a good qualityconversion film is made possible even in the absence of oxidizing agentby holding the pH and chelating agent concentration within the rangesspecified above.

Another crucial point in the treatment process in accordance with thepresent invention is that the oxidation-reduction potential of thetreatment bath is to be controlled to ≦450 mV during treatment. Nospecific restrictions apply to the electrodes used to measure theoxidation-reduction potential. The potentials provided in the presentinvention were obtained using a platinum electrode as theoxidation-reduction electrode and a silver-saturated silver chlorideelectrode as the reference electrode. When the oxidation-reductionpotential is ≦450 mV during this measurement, the iron ion is presentalmost entirely in the divalent state and the production of sludge isinhibited.

In addition to deliberately added oxidizing agent, atmospheric oxygenalso can oxidize the divalent iron ions in the treatment bath. Thetendency for the divalent iron ions to be oxidized by atmospheric oxygenvaries as a function of the precise nature of the equipment, the sprayconditions, and the like. The oxidation-reduction potential may in somecases exceed 450 mV when the present invention is implemented on acontinuous basis under conditions in which air tends to be taken up andthe difficult-to-avoid removal of bath by the treatment substraterequires only minor renewal of the surface treatment bath. Becausesludge will be produced under such circumstances and quality andequipment maintenance will then again become problematic, reducing agentmust be added on a preliminary basis or when the oxidation-reductionpotential becomes elevated in order thereby to maintain theoxidation-reduction potential at ≦450 mV. No specific restrictions applyto this reducing agent, but substances that inhibit conversion filmformation on the tinplate by the surface treatment bath should beavoided. Viewed from this perspective, phosphorous acid andhypophosphorous acid are preferred as reducing agents, because the maincomponent of the surface treatment bath is phosphate ion and bothphosphorous acid and hypophosphorous acid are converted into phosphateion in fulfilling their function as reducing agent. Thus, adverseeffects due to an accumulation of their decomposition product arecompletely avoided.

Phosphorous acid and hypophosphorous acid can be added as the acid orsalt. The quantity of addition will vary as a function of the treatmentconditions, but is preferably as small as possible from the standpointof economics. Thus, the presence or addition of the minimum quantitythat maintains the oxidation-reduction potential at ≦450 mV issufficient. In other words, the quantity of addition of the reducingagent can be regulated based on the oxidation-reduction potential. Whenthe reducing agent is supplied so as to maintain the oxidation-reductionpotential at ≦450 mV, substantially all of the iron ions in thecomposition are maintained in the divalent state and the production ofsludge in the surface treatment bath can be prevented even duringcontinuous treatment over long periods of time.

The conversion film that is formed will now be briefly considered. Theconversion film that is formed by a phosphate surface treatment bath fortinplate is generally a phosphate salt whose principal component is tinphosphate, and the basic mechanism for its formation is believed to bethe same even for the present invention. Thus, the tinplate substrate isetched by the phosphate ions and chelating agent (particularly condensedphosphate ions); a local increase in the pH at the interface occurs atthis time; and a phosphate conversion film (principally of tinphosphate) precipitates on the surface.

One difference between prior phosphate films and the phosphate film ofthe present invention is the fact that the prior films are produced inthe presence of chelating agent and oxidizing agent while in the presentinvention production occurs in the presence of chelating agent and(optionally) reducing agent, i.e., the iron ions are only in thedivalent state and production occurs essentially in the absence oftrivalent ferric ions. A second difference is that the "sludge skin" isthen presumably negligible for the film of the present invention."Sludge skin" refers to the adhesion of a relatively poorly adherent,sediment-like substance in the vicinity of the tin phosphate filmproper. Moreover, because the phosphate film formed on tin-plated steelsheet in the case of tinplate DI can is usually extremely thin,approximately 10 to 20 Ångstroms, in both the tin-plated regions and theexposed iron regions, the sludge skin is not susceptible in this case tovisual evaluation, in contrast to ordinary zinc phosphate films, forwhich the areal density is approximately 1 to 10 g/m² and thecorresponding thickness from 1,000 to 8,000 Ångstroms. The exactsituation has therefore yet to be elucidated.

The treatment of tinplate using the surface treatment bath of thepresent invention is briefly explained below. The treatment bath of thepresent invention is used, preferably as part of the following sequence,which is provided as a preferred example:

Tinplate cleaning: degreasing (a weakly alkaline degreaser is typicallyused)

Water wash

Surface treatment (application of treatment bath of the presentinvention)

Treatment temperature: 30° C. to 70° C.

Treatment technique: spray or immersion

Treatment time: 2 to 40 seconds

Water wash

Wash with de-ionized water

Drying.

The treatment temperature with the surface treatment bath of the presentinvention is preferably 30° C. to 70° C., and heating the bath generallyto 40° C. to 60° C. for use is particularly preferred. The preferredtreatment time is 2 to 40 seconds. At below 2 seconds, the reaction isinadequate and a highly corrosion-resistant film will not normally beformed. On the other hand, the performance does not improve at treatmenttimes in excess of 40 seconds, and therefore optimal treatment timesfall in the range of 2 to 40 seconds.

While the treatment technique can be either immersion or spray, asdiscussed above the present invention gives particularly good effectswhen used with spray equipment.

As discussed hereinbefore, the oxidation state of the iron ions thathave eluted from the tinplate significantly affects sludge production.Iron ions are believed to elute from the tinplate as divalent ferrousions. In the treatment bath in accordance with the present invention,the iron ions are typically present as ferrous ions at a concentrationof about 0.005 to about 0.025 g/L when the line is running, while ferricions are essentially not present. In contrast to this, the ferrous ionsare almost entirely oxidized in prior art treatment baths to yieldferric ions or colloid in a concentration typically on the level of 0.05g/L. Sludge is produced because this ferric ion and the phosphate ionform an insoluble salt that also traps the tin and phosphate ions thatare present. In other words, sludge production in the surface treatmentbath can be suppressed by maintaining the iron ion eluted from thetinplate in the divalent state.

By essentially omitting the oxidizing agent that has been used inprior-art treatment baths, the iron ions in the present inventionconsist almost completely of divalent iron ions. It is thought that thisoccurs because both divalent tin ions and tetravalent tin ions arepresent and the divalent tin ions rapidly reduce trivalent iron ions todivalent iron ions.

The oxidation-reduction potential of a composition is measured by theequilibrium electrode potential of an inert oxidation-reductionelectrode in contact with the composition, and it represents themagnitude of the oxidizing power or reducing power of the composition.The following equation gives the oxidation-reduction potential E_(e) forthe half-reaction oxidation of ferrous ion to ferric ion according tothe chemical equation Fe²⁺ →Fe³⁺ +e⁻.

    E.sub.e =E.sub.o --(RT/ In([Fe.sup.+2 ]/[Fe.sup.+3 ]),

where R=the gas constant, T=the absolute temperature, =Faraday'sconstant, square brackets indicate activities of the chemical specieswithin the brackets, and E_(o) =the standard electrode potential for thereaction. Larger values of E_(e) correspond to a higher oxidizing powerand thus to a higher ferric ion/ferrous ion ratio; smaller values of theoxidation-reduction potential indicate fewer ferric ions. Accordingly,the average oxidation state of the eluted iron ions can be controlled bycontrolling the oxidation-reduction potential.

EXAMPLES

The utility of the surface treatment bath of the present invention isexplained below through a comparison of several working examples withcomparison examples. In these examples, the tinplate substratesconsisted of tinplate DI cans fabricated by the DI processing oftin-plated steel sheet. The corrosion resistance after surface treatmentwas evaluated using the iron exposure value ("IEV"). The IEV wasmeasured in accordance with U.S. Pat. No. 4,332,646. Lower IEV valuescorrespond to a better corrosion resistance, and values ≦150 generallycorrespond to an excellent corrosion resistance.

The paint adherence was evaluated through the peel strength. Anepoxy/urea can paint was coated on the surface of the treated can to apaint film thickness of 5 to 7 micrometers ("μM") followed by baking for4 minutes at 215° C. Each can was subsequently cut into 5×150 mm strips,and a test specimen was prepared by hot pressing polyamide film onto astrip. The test specimen was then peeled in a 180° peel test and thepeel strength was measured. In this case, larger peel strength valuesindicate a better paint adherence, and values of 1.5 kilograms force("kgf")/5 mm-width or more are generally regarded as excellent.

Sludge production was evaluated as follows. 0.05 g/L of iron ions fromferrous chloride was added to the particular surface treatment bath asdescribed in the working or comparison example, the pH was adjusted, thebath was allowed to stand for 1 day, and the status of the bath was theninspected. A bath that was transparent and free of precipitate or thelike was judged as essentially free of ferric ion. Theoxidation-reduction potential was measured after standing using aplatinum electrode as the oxidation-reduction electrode and asilver-saturated silver chloride electrode as the reference electrode.

In order to evaluate sludge production during continuous treatment, acontinuous treatment was run using freshly prepared surface treatmentbath as reported in the particular example or comparison example. Thecontinuous treatment used 2 liters ("L") of treatment bath, and a30-second treatment was conducted on a total of 360 cans. The bathquantity and pH were maintained at their initial values through theaddition of the particular surface treatment bath and phosphoric acid,respectively. The bath status and oxidation-reduction potential ("ORP")were evaluated after the continuous test.

A bath that was transparent and free of precipitate or the like wasjudged to be essentially free of ferric ion. In addition, the iron ionconcentration in the treatment bath after continuous treatment wasmeasured by atomic absorption. When a precipitate had been produced,analysis was run by dissolving the precipitate by the addition ofhydrochloric acid.

EXAMPLE 1

Tinplate DI cans (fabricated by the DI processing of tin-plated steelsheet) were (1) thoroughly cleaned using a hot 1% aqueous solution of aweakly alkaline degreaser (FINECLEANER™ 4488 from Nihon ParkerizingCompany, Limited); (2) sprayed for 20 seconds with surface treatmentbath 1 heated to 60° C.; (3) washed with tap water; (4) sprayed withdeionized water (with a specific resistance ≧3 Mohm-cm) for 10 seconds;and (5) dried in a hot-air drying oven for 3 minutes at 180° C. Thetreated cans were evaluated for corrosion resistance and paintadherence, and surface treatment bath 1 was evaluated for sludgeproduction.

    ______________________________________                                        Surface treatment bath 1                                                      ______________________________________                                        75% phosphoric acid (H.sub.3 PO.sub.4)                                                           10.0 gL (PO.sub.4.sup.3- : 7.2 g/L)                        Sodium pyrophosphate                                                                              1.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.4 gL)                 (Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O)                                        SnCl.sub.4.5H.sub.2 O                                                                             0.6 g/L (Sn.sup.3+ : 0.2 g/L)                             FeCl.sub.3.6H.sub.2 O                                                                             4.8 mg/L (Fe.sup.3+ : 1.0 mg/L)                           Phosphorous acid (H.sub.3 PO.sub.3)                                                              0.01 g/L                                                   pH 3.0 (adjusted with sodium carbonate)                                       ______________________________________                                    

The ferric chloride was added in order to examine the effect oftrivalent iron ion on sludge production.

EXAMPLE 2

Tinplate DI can was cleaned using the same conditions as in Example 1,sprayed for 10 seconds with surface treatment bath 2 heated to 40° C.,and then washed with water and dried under the same conditions as inExample 1. The treated can was evaluated for corrosion resistance andpaint adherence, and surface treatment bath 2 was evaluated for sludgeproduction.

    ______________________________________                                        Surface treatment bath 2                                                      ______________________________________                                        75% phosphoric acid (H.sub.3 PO.sub.4)                                                            5.0 gL (PO.sub.4.sup.3- : 3.6 g/L)                        Sodium pyrophosphate                                                                              2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L)                (Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O)                                        SnCl.sub.4.5H.sub.2 O                                                                             1.2 g/L (Sn.sup.4+ : 0.4 g/L)                             pH 2.8 (adjusted with phosphoric acid)                                        ______________________________________                                    

EXAMPLE 3

Tinplate DI can was cleaned using the same conditions as in Example 1,sprayed for 40 seconds with surface treatment bath 3 heated to 60° C.,and then washed with water and dried under the same conditions as inExample 1. The treated can was evaluated for corrosion resistance andpaint adherence, and surface treatment bath 3 was evaluated for sludgeproduction.

    ______________________________________                                        Surface treatment bath 3                                                      ______________________________________                                        75% Phosphoric acid (H.sub.3 PO.sub.4)                                                             5.0 g/L (PO.sub.4.sup.3- : 3.6 g/L)                      Sodium pyrophosphate                                                                               2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L)               (Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O)                                        SnCl.sub.4.5H.sub.2 O                                                                             0.10 g/L (Sn.sup.4+ : 0.03 g/L)                           Hypophosphorous acid (H.sub.3 PO.sub.2)                                                           0.01 g/L                                                  pH 4.0 (adjusted with sodium hydroxide)                                       ______________________________________                                    

EXAMPLE 4

Tinplate DI can was cleaned using the same conditions as in Example 1,sprayed for 10 seconds with surface treatment bath 4 heated to 40° C.,and then washed with water and dried under the same conditions as inExample 1. The treated can was evaluated for corrosion resistance andpaint adherence, and surface treatment bath 4 was evaluated for sludgeproduction.

    ______________________________________                                        Surface treatment bath 4                                                      ______________________________________                                        75% Phosphoric acid (H.sub.3 PO.sub.4)                                                           15.0 g/L (PO.sub.4.sup.3- : 10.8 g/L)                      Sodium pyrophosphate                                                                              2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L)                (Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O)                                        Sodium tripolyphosphate                                                                           1.0 g/L (P.sub.3 O.sub.10.sup.5- : 0.6 g/L)               (Na.sub.5 P.sub.3 O.sub.10)                                                   SnCl.sub.4.5H.sub.2 O                                                                             1.2 g/L (Sn.sup.4+ : 0.4 g/L)                             Phosphorous acid (H.sub.3 PO.sub.3)                                                              0.01 g/L                                                   Hypophosphorous acid (H.sub.3 PO.sub.2)                                                          0.01 g/L                                                   pH 3.0 (adjusted with sodium carbonate)                                       ______________________________________                                    

EXAMPLE 5

Tinplate DI can was cleaned using the same conditions as in Example 1,sprayed for 30 seconds with surface treatment bath 5 heated to 50° C.,and then washed with water and dried under the same conditions as inExample 1. The treated can was evaluated for corrosion resistance andpaint adherence, and surface treatment bath 5 was evaluated for sludgeproduction.

    ______________________________________                                        Surface treatment bath 5                                                      ______________________________________                                        75% Phosphoric acid (H.sub.3 PO.sub.4)                                                            1.0 g/L (PO.sub.4.sup.3- : 0.7 g/L)                       Sodium pyrophosphate                                                                              2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L)                (Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O)                                        SnCl.sub.4.5H.sub.2 O                                                                             1.2 g/L (Sn.sup.4+ : 0.4 g/L)                             Phosphorous acid (H.sub.3 PO.sub.3)                                                              0.01 g/L (H.sub.3 PO.sub.3 : 0.01 g/L)                     pH 3.0 (adjusted with phosphoric acid)                                        ______________________________________                                    

EXAMPLE 6

Tinplate DI can was cleaned using the same conditions as in Example 1,sprayed for 20 seconds with surface treatment bath 6 heated to 50° C.,and then washed with water and dried under the same conditions as inExample 1. The treated can was evaluated for corrosion resistance andpaint adherence, and surface treatment bath 6 was evaluated for sludgeproduction.

    ______________________________________                                        Surface treatment bath 6                                                      ______________________________________                                        75% Phosphoric acid (H.sub.3 PO.sub.4)                                                            5.0 g/L (PO.sub.4.sup.3- : 3.6 g/L)                       Sodium pyrophosphate                                                                              2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L)                (Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O)                                        Tin (by dissolution of tin metal)                                                                 0.2 g/L (Sn.sup.2+ : 0.2 g/L)                             Phosphorous acid (H.sub.3 PO.sub.3)                                                              0.01 g/L (H.sub.3 PO.sub.3 : 0.01 g/L)                     pH 3.0 (adjusted with phosphoric acid)                                        ______________________________________                                    

EXAMPLE 7

Tinplate DI can was cleaned using the same conditions as in Example 1,sprayed for 2 seconds with surface treatment bath 7 heated to 70° C.,and then washed with water and dried under the same conditions as inExample 1. The treated can was evaluated for corrosion resistance andpaint adherence, and surface treatment bath 7 was evaluated for sludgeproduction.

    ______________________________________                                        Surface treatment bath 7                                                      ______________________________________                                        75% Phosphoric acid (H.sub.3 PO.sub.4)                                                           30.0 g/L (PO.sub.4.sup.3- : 21.6 g/L)                      Sodium pyrophosphate                                                                              2.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.8 g/L)                (Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O)                                        Sodium tripolyphosphate                                                                           1.0 g/L (P.sub.3 O.sub.10.sup.5- : 0.6 g/L)               (Na.sub.5 P.sub.3 O.sub.10)                                                   SnCl.sub.4.5H.sub.2 O)                                                                            1.2 g/L (Sn.sup.4+ : 0.4 g/L)                             Phosphorous acid (H.sub.3 PO.sub.3)                                                              0.01 g/L                                                   Hypophosphorous acid (H.sub.3 PO.sub.2)                                                          0.01 g/L                                                   pH 2.0 (adjusted with phosphoric acid)                                        ______________________________________                                    

Comparison Example 1

Tinplate DI can was cleaned using the same conditions as in Example 1,sprayed for 30 seconds with surface treatment bath 8 heated to 40° C.,and then washed with water and dried under the same conditions as inExample 1. The treated can was evaluated for corrosion resistance andpaint adherence, and surface treatment bath 8 was evaluated for sludgeproduction.

    ______________________________________                                        Surface treatment bath 8                                                      ______________________________________                                        75% Phosphoric acid (H.sub.3 PO.sub.4)                                                            10.0 g/L (PO.sub.4.sup.3- : 7.2 g/L)                      SnCl.sub.4.5H.sub.2 O                                                                              0.6 g/L (Sn.sup.4+ : 0.2 g/L)                            Phosphorous acid (H.sub.3 PO.sub.3)                                                               0.01 g/L                                                  pH 3.0 (adjusted with sodium carbonate)                                       ______________________________________                                    

Comparison Example 2

Tinplate DI can was cleaned using the same conditions as in Example 1,sprayed for 30 seconds with surface treatment bath 9 heated to 50° C.,and then washed with water and dried under the same conditions as inExample 1. The treated can was evaluated for corrosion resistance andpaint adherence, and surface treatment bath 9 was evaluated for sludgeproduction.

    ______________________________________                                        Surface treatment bath 9                                                      ______________________________________                                        75% Phosphoric acid (H.sub.3 PO.sub.4)                                                            10.0 g/L (PO.sub.4.sup.3- : 7.2 g/L)                      Sodium pyrophosphate                                                                               1.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.4 g/L)               (Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O)                                        SnCl.sub.4.5H.sub.2 O                                                                              0.6 g/L (Sn.sup.4+ : 0.2 g/L)                            Phosphorous acid (H.sub.3 PO.sub.3)                                                               0.01 g/L                                                  pH 4.6 (adjusted with sodium hydroxide)                                       ______________________________________                                    

Comparison Example 3

Tinplate DI can was cleaned using the same conditions as in Example 1,sprayed for 30 seconds with surface treatment bath 10 heated to 50° C.,and then washed with water and dried under the same conditions as inExample 1. The treated can was evaluated for corrosion resistance andpaint adherence, and surface treatment bath 10 was evaluated for sludgeproduction.

    ______________________________________                                        Surface treatment bath 10                                                     ______________________________________                                        75% Phosphoric acid (H.sub.3 PO.sub.4)                                                           1.33 g/L (PO.sub.4.sup.3- : 0.97 g/L)                      Sodium pyrophosphate                                                                              1.0 g/L (P.sub.2 O.sub.7.sup.4- : 0.4 g/L)                (Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O)                                        SnCl.sub.4.5H.sub.2 O                                                                             0.6 g/L (Sn.sup.4+ : 0.2 g/L)                             FeCl.sub.3.6H.sub.2 O)                                                                             48 mg/L (Fe.sup.3+ : 10 mg/L)                            pH 4.0 (adjusted with sodium carbonate)                                       ______________________________________                                    

Comparison Example 4

Tinplate DI can was cleaned using the same conditions as in Example 1and was then sprayed for 30 seconds with a 4% aqueous solution (heatedto 50° C.) of a commercial tinplate DI can surface treatment agent(PALFOS™ K3466 from Nihon Parkerizing Company, Limited). This wasfollowed by washing with water and drying under the same conditions asin Example 1. The treated can was evaluated for corrosion resistance andpaint adherence, and the treatment bath was evaluated for sludgeproduction.

Comparison Example 5

Tinplate DI can was cleaned using the same conditions as in Example 1and was then sprayed for 30 seconds with a 4% aqueous solution (heatedto 50° C.) of a commercial tinplate DI can surface treatment agent(PALFOS™ K3482 from Nihon Parkerizing Company, Limited). This wasfollowed by washing with water and drying under the same conditions asin Example 1. The treated can was evaluated for corrosion resistance andpaint adherence, and the treatment bath was evaluated for sludgeproduction.

The results are reported in Table 1.

Benefits of the Invention

As discussed in the preceding, treating the surface of tinplate(tin-plated steel) sheet, strip, or shaped objects (cans or the like)with the surface treatment bath of the present invention accrues thehighly desirable effects of imparting an excellent corrosion resistanceand adherence to the tinplate surface and avoiding sludge production inthe treatment bath when treatment is run on a continuous basis.

                                      TABLE 1                                     __________________________________________________________________________    Example ("E")                                                                 or Com-                                                                       parison Conversion Film Quality                                                                    Sludge Production Results                                Example Corrosion    With Direct Addition of Iron                                                                After Continuous Use for Treatment         ("CE")  Resistance                                                                           Adhesion,                                                                           Bath          Bath                                       Number  (IEV Value)                                                                          kgf/5 mm                                                                            Appearance                                                                           ORP, mV                                                                              Appearance                                                                           ORP, mV                                                                             Iron, ppm                     __________________________________________________________________________    E 1     100    3.0   Transparent                                                                          370    Transparent                                                                          170   18                            E 2     100    3.0   Transparent                                                                          430    Transparent                                                                          260    9                            E 3     120    2.5   Transparent                                                                          420    Transparent                                                                          210   10                            E 4     100    3.0   Transparent                                                                          400    Transparent                                                                          200    8                            E 5     100    3.0   Transparent                                                                          390    Transparent                                                                          200   12                            E 6     100    3.0   Transparent                                                                          400    Transparent                                                                          210   18                            E 7     150    2.0   Transparent                                                                          430    Transparent                                                                          230   25                            CE 1    500    1.5   Tur. W. Ppt.                                                                         450    Tur. W. Ppt.                                                                         650   80                            CE 2    300    1.5   Transparent                                                                          400    Transparent                                                                          350    8                            CE 3    300    1.5   W. Turbidity                                                                         550    Tur. W. Ppt.                                                                         650   30                            CE 4    100    3.0   Tur. W. Ppt.                                                                         700    Tur. W. Ppt.                                                                         700   60                            CE 5    100    3.0   Tur. W. Ppt.                                                                         700    Tur. W. Ppt.                                                                         700   25                            __________________________________________________________________________     Notes for Table 1                                                             "ORP" means "oxidationreduction potential", which was measured against a      silversaturated silve chloride reference electrode; "IEV" means: "iron        exposure value", as described in the main text; "Tur. W. Ppt." means          "Turbid White Precipitate"; "W. Turbid." means "white turbidity".        

The invention claimed is:
 1. An aqueous liquid composition suitable for treating tinplate surfaces, said composition having a pH in the range from 2.0 to 4.5 and an oxidation- reduction potential not greater than 450 mV more oxidizing than a silver-saturated silver chloride reference electrode, and consisting essentially of water and:(A) from 1 to 30 g/L of phosphate ions, (B) from 0.1 to 5.0 g/L of chelating agent, and (C) from 0.01 to 2.0 g/L of tin ions.
 2. A composition according to claim 1, wherein the chelating agent is selected from condensed phosphate ions.
 3. A composition according to claim 2, wherein the concentration of tin ions is from 0.1 to 1.0 g/L and the pH is from 2.5 to 3.5.
 4. A composition according to claim 3, wherein the concentration of tin ions is from 0.2 to 0.6 g/L and the pH is from 2.7 to 3.3.
 5. A composition according to claim 1, wherein the concentration of tin ions is from 0.1 to 1.0 g/L and the pH is from 2.5 to 3.5.
 6. A composition according to claim 5, wherein the concentration of tin ions is from 0.2 to 0.6 g/L and the pH is from 2.7 to 3.3.
 7. A composition according to claim 6, wherein the concentration of phosphate ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L.
 8. A composition according to claim 5, wherein the concentration of phosphate ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L.
 9. A composition according to claim 4, wherein the concentration of phosphate ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L.
 10. A composition according to claim 3, wherein the concentration of phosphate ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L.
 11. A composition according to claim 2, wherein the concentration of phosphate ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L.
 12. A composition according to claim 1, wherein the concentration of phosphate ions is within the range from 4 to 8 g/L and the concentration of chelating agent is within the range from 0.2 to 1.0 g/L.
 13. A process for treating tinplate to form a protective coating thereon, comprising contacting the tinplate with a composition according to claim 12 at a temperature within the range from 30° to 70° C. for a time within the range from 2 to 40 seconds.
 14. A process according to claim 13, wherein a reducing agent is added to the initial composition as the latter is used, in an amount sufficient to maintain the oxidation-reduction potential of the composition not more than 450 mV more oxidizing than a silver-saturated silver chloride reference electrode.
 15. A process according to claim 14, wherein the reducing agent is selected from the group consisting of phosphorous acid, hypophosphorous acid, their salts, and mixtures of any two or more of said acids and their salts.
 16. A process for treating tinplate to form a protective coating thereon, comprising contacting the tinplate with a composition according to claim 7 at a temperature within the range from 40° to 60° C. for a time within the range from 2 to 40 seconds.
 17. A process according to claim 16, wherein a reducing agent is added to the initial composition as the latter is used, in an amount sufficient to maintain the oxidation-reduction potential of the composition not more than 450 mV more oxidizing than a silver-saturated silver chloride reference electrode.
 18. A process according to claim 17, wherein the reducing agent is selected from the group consisting of phosphorous acid, hypophosphorous acid, their salts, and mixtures of any two or more of said acids and their salts.
 19. A process for treating tinplate to form a protective coating thereon, comprising contacting the tinplate with a composition according to claim 1 at a temperature within the range from 30° to 70° C. for a time within the range from 2 to 40 seconds.
 20. A process according to claim 19, wherein a reducing agent is added to the initial composition as the latter is used, in an amount sufficient to maintain the oxidation-reduction potential of the composition not more than 450 mV more oxidizing than a silver-saturated silver chloride reference electrode. 